Selection of
an ascent abort mode may become necessary if there is a failure
that affects vehicle performance, such as the failure of a main
engine or an orbital maneuvering system failure. Other failures
dictating early termination of a flight, such as a cabin leak, might
require the selection of an abort mode.

There are two
basic types of ascent abort modes for space shuttle missions: intact
aborts and contingency aborts. An intact abort would provide a safe
return of the orbiter to a planned landing site, while a contingency
abort is designed to permit the crew to survive following more severe
failures when an intact abort is not possible. A contingency abort
would generally result in a ditch operation.

There are four
types of intact abort modes: return to launch site, transatlantic
landing, abort to orbit and abort once around.

The ATO mode
is designed to allow the vehicle to achieve a temporary orbit that
is lower than the nominal orbit. This mode requires less performance
and permits time to evaluate problems and to choose either an early
deorbit burn or an OMS maneuver to raise the orbit and continue
the mission.

The AOA would
permit the vehicle to fly once around the Earth and make a normal
entry and landing. This mode generally involves two OMS burns, with
the second burn being a deorbit maneuver. There are two types of
AOA trajectories: a normal AOA and a shallow AOA (which is considered
only for contingency aborts). The entry trajectory for the normal
AOA is similar to the nominal entry trajectory. The shallow AOA
results in a flatter entry trajectory, which is less desirable than
that of the normal AOA but uses less propellant in the OMS maneuvers.
The shallow entry trajectory is less desirable because it exposes
the vehicle to a longer period of atmospheric heating and to less
predictable aerodynamic drag forces.

The TLA abort
is designed to permit an intact landing on the other side of the
Atlantic Ocean. This mode would result in a ballistic trajectory,
which does not require an OMS maneuver.

The RTLS would
require flying downrange to dissipate propellant and turning around
under power to return directly to a landing at or near the launch
site.

There is a
definite order of preference for the various abort modes. The type
of failure and the time of the failure would determine which type
of abort is selected. In cases where performance loss is the only
factor, the preferred modes, in order, would be ATO, AOA, TLA and
RTLS. The mode chosen would be the highest one that could be completed
with the remaining vehicle performance. For certain support system
failures, such as cabin leaks or vehicle cooling problems, the preferred
mode might be the one that would end the mission most quickly. In
these cases, TLA or RTLS might be preferable to AOA or ATO. A contingency
abort would never be chosen if another abort option existed.

The Mission
Control Center is primarily responsible for calling aborts, since
the controllers have more precise knowledge of the orbiter's state
vector (through the use of sophisticated tracking equipment and
ground computer resources) than the crew can obtain from the onboard
navigation system. Before MECO, Mission Control periodically calls
the crew to tell them which abort mode is (or is not) available.
If ground communications are lost, the flight crew has onboard methods,
such as cue cards, dedicated displays and GN&C; CRT display information,
from which to determine the current abort region.

The abort mode
selected would depend on the cause and timing of the failure causing
the abort and on which mode is safest or improves the chances for
mission success. If the emergency is a main engine failure, the
flight crew and MCC would select the best option available at the
time of the failure. If the problem is a system failure that jeopardizes
the vehicle, the abort mode that would result in the earliest vehicle
landing would be chosen. RTLS and TLA would be the quickest options
(35 minutes), whereas an AOA would require approximately 90 minutes.
Which of these is selected would depend on the time of the failure
with three good main engines.

The flight
crew would select the abort mode by positioning the abort mode switch
on panel F6 and depressing the abort push button on the panel. This
switch is a rotary model with off , RTLS , TAL-AOA-S (shallow) and
ATO positions.